Allow loading mod maps only

[L-P]

Add sv_load_mod_maps, contrary to sv_load_all_maps it only loads
mod-specific maps to avoid loading 125 extra maps if you base your mod on
valve and only care about your own maps.

BC break: for it to make sense I had to rename sv_stop_loading_all_maps to
sv_stop_loading_maps. I could add a sv_stop_loading_mod_maps instead.